Common Cause Failures: What Your FMEA Must Include

Redundancy only works when failures are independent. Common cause failures (CCFs) — where multiple channels fail for the same reason — can silently erode your safety integrity if not addressed in design and FMEA.

Typical Common Causes

• Shared power supply or grounding faults.
• Mechanical vibration affecting multiple sensors.
• Incorrect configuration copied across redundant controllers.
• Environmental stress (temperature, dust, humidity) beyond spec.

Quantifying CCF Risk

Both ISO 13849 and IEC 62061 include guidance for scoring CCF using the β-factor method. A β of 0.05 (5%) is achievable with physical separation, diversity, and good maintenance.

Design Mitigations

• Separate power supplies and routing paths.
• Use different sensor technologies (optical + mechanical).
• Apply independent software validation or CRC checks.
• Conduct environmental testing to verify robustness.

Example

An automotive press line added isolation between redundant light curtains after detecting EMI interference as a shared failure path. The updated FMEA reduced β from 0.15 to 0.05.

Related Articles

• Safety PLCs vs Relays: When Each Makes Sense
• PL and SIL Without Tears: Selecting Safety Functions
• Proof Testing Intervals That Don’t Kill Uptime

Conclusion

Redundancy isn’t safety unless CCF is under control. Your FMEA must prove independence, not just duplication.